Methods and devices for determination of arrhythmia rate zone thresholds

ABSTRACT

Approaches for determining threshold values for one or more arrhythmia rate zones and/or the number of rate zones are described. A probability function for heart rate is determined using collected and measured heart rate values. One or more heart rate probability values are selected. Thresholds for arrhythmia rate zones are determined from the probability function based on the selected probability values. Determining the rate zone thresholds may involve determining a threshold for a lower rate limit and/or determining one or more tachyarrhythmia rate zone thresholds. The number of rate zones may also be determined based on the probability function.

FIELD OF THE INVENTION

The present invention relates generally to determining rate zones fortachyarrhythmia detection and therapy.

BACKGROUND OF THE INVENTION

Proper cardiac function relies on the synchronized contractions of theheart at regular intervals. When the heart is functioning normally,synchronized cardiac contractions are initiated at the sinoatrial nodeand the heart is said to be operating in normal sinus rhythm. However,if contractions of the heart become irregular or uncoordinated, or ifthe contraction rate is too fast or too slow, the heart rhythm isdescribed as arrhythmic. Cardiac arrhythmia may be caused, for example,by disease processes or from aberrant electrical conduction patternsoccurring in the heart tissue. Cardiac arrhythmia impairs cardiacpumping efficiency and some types of cardiac arrhythmia can be lifethreatening.

A cardiac arrhythmia that originates in a region of the heart above theventricles is denoted a supraventricular tachyarrhythmia (SVT). Atrialfibrillation and atrial flutter are examples of SVT. Both conditions arecharacterized by rapid, uncoordinated contractions of the atria.

Another example of SVT is sinus tachycardia, which is an increased heartrate due to exercise or a quick emotional response. In contrast toatrial fibrillation and atrial flutter, sinus tachycardia ischaracterized by rapid, coordinated contractions of the atria,compensating for the increased strain placed upon the body duringexercise or quick emotional responses. Whereas atrial fibrillation andatrial flutter are “abnormal” (yet not lethal), sinus tachycardia is“normal” (and also not lethal).

Cardiac arrhythmias originating in a ventricular region of the heart aredenoted ventricular tachyarrhythmia. Ventricular tachyarrhythmia (VT) ischaracterized by rapid ventricular contractions and can degenerate intoventricular fibrillation (VF). Ventricular fibrillation producesextremely rapid, non-coordinated contractions of the ventricles.Ventricular fibrillation is fatal unless the heart is returned to sinusrhythm within a few minutes.

Implantable cardiac rhythm management (CRM) devices, includingpacemakers and implantable cardioverter/defibrillators, have been usedto deliver effective treatment to patients with serious cardiacarrhythmias. Cardiac rhythm management devices may treat cardiacarrhythmias with a variety of tiered therapies. These tiered therapiesrange from delivering low energy pacing pulses timed to assist the heartin maintaining pumping efficiency to providing high-energy shocks totreat and/or terminate fibrillation. To effectively deliver thesetreatments, the CRM device must first identify the type of arrhythmiathat is occurring, after which appropriate therapy may be delivered tothe heart.

SUMMARY OF THE INVENTION

The present invention is directed to methods and systems used fortachyarrhythmia detection and therapy. One embodiment of the inventionis directed to a method for detecting cardiac arrhythmia. Heart ratevalues are collected and measured. A probability function for heart rateis determined using the collected and measured heart rate values. One ormore heart rate probability values are selected. One or more thresholdsfor arrhythmia rate zones are determined from the probability functionbased on the one or more probability values. For example, determiningthe one or more rate zone thresholds may involve determining a thresholdfor a lower rate limit and/or determining one or more tachyarrhythmiarate zone thresholds. Cardiac tachyarrhythmia may be detected and/ortherapy delivered based on the rate zone thresholds.

In accordance with aspects of the invention, the collected and measuredheart rate values comprise RR or PP intervals. The probability functionis determined using the RR or PP intervals. A probability functiondeveloped using RR intervals may be used to determine one or moreventricular rate zone thresholds. A probability function developed usingPP intervals may be used to determine one or more atrial rate zonethresholds.

In one implementation, the probability function comprises a cumulativedensity function.

The probability values may be selected by entering one or morefractional probability values via a patient-external device.

The probability function may be used as a lookup table with the one ormore probability values used as indexes to the lookup table. Theprobability values may be input into the probability function todetermine the one or more rate thresholds.

In one implementation, a number of arrhythmia rate zones may bedetermined based on the probability function.

The probability function may involve a one dimensional probabilityfunction of ventricular rate values or a one dimensional probability ofatrial rate values. In another example, the probability function mayinvolve a two dimensional probability function of atrial rate values andventricular rate values. Atrioventricular patterns may be identifiedbased on the two dimensional probability function of atrial rate valuesand ventricular rate values. The one or more rate zone thresholds may bedetermined based on the two dimensional probability function.

Multiple rate thresholds respectively associated with multiple ratezones may be determined. An arrhythmia may be detected based on themultiple rate zones; and cardiac stimulation therapy delivered to treatthe detected arrhythmia. For example, the cardiac stimulation therapymay involve delivery of multiple therapies, each therapy respectivelyassociated with one of the multiple rate zones.

Another embodiment of the invention is directed to a cardiac device. Thecardiac device includes sensing circuitry that senses cardiac electricalsignals. Heart rate measurement circuitry measures heart rate valuesbased on the sensed cardiac electrical signals. A memory buffer storesthe measured heart rate values. A rate zone processor develops aprobability function for heart rate based on the measured heart ratevalues and determines at least one rate zone threshold from theprobability function based a heart rate probability value. Theprobability function may comprise a probability density function or acumulative distribution function, for example. The at least one ratezone threshold may comprise a lower rate limit and/or may comprise oneor more ventricular and/or atrial tachyarrhythmia rate zone thresholds.

The cardiac device may further include detection circuitry configured todetect arrhythmia based on the rate zone threshold and therapy circuitryconfigured to deliver electrical stimulation therapy to treat thearrhythmia.

In one implementation, the rate zone processor is configured toautomatically select the heart rate probability. In one implementation,the cardiac device includes communication circuitry configured toreceive the heart rate probability value entered via a patient-externaldevice.

The rate zone processor may use the probability function as a lookuptable to determine the rate zone threshold using the heart rateprobability value as an index. The rate zone processor may solve theprobability function using the heart rate probability value to determinethe rate zone threshold.

In one configuration, the heart rate measurement circuitry measuresintervals between cardiac beats. The rate zone processor develops theprobability function based on the measured intervals.

According to one implementation, the rate zone processor may be furtherconfigured to determine a number of rate zones based on the probabilityfunction.

Another embodiment of the invention is directed to a method fordetermining rate zones for cardiac arrhythmia. The method includescollecting measured heart rate values. A probability function for thecollected and measured heart rate values is determined. A number of ratezones for tachyarrhythmia detection is determined based on theprobability function.

In one approach, the number of rate zones may be determined based anumber of features of the probability function.

In one approach, the probability function is a cumulative distributionfunction. The number of rate zones is determined by analyzing thecumulative distribution function to identify flat portions of thecumulative distribution function.

In another approach, the probability function is a probability densityfunction. The number of rate zones is determined by analyzing theprobability density function to identify peaks of the probabilitydensity function. The number of rate zones may be updated periodically.

Another embodiment of the invention is directed to a cardiac device. Thedevice includes sensing circuitry configured to sense cardiac electricalsignals. Heart rate measurement circuitry collects measured heart ratevalues based on the sensed cardiac electrical signals. A rate zoneprocessor determines a number of rate zones for tachyarrhythmiadetection based on a probability function for the collected and measuredheart rate values.

In one approach, the probability function is a cumulative distributionfunction. The rate zone processor is configured to analyze thecumulative distribution function to identify flat portions of thecumulative distribution function and to determine the number of ratezones for tachyarrhythmia detection based on the identified flatportions of the cumulative distribution function. In another approach,the probability function is a probability density function. The ratezone processor analyzes the cumulative distribution function to identifypeaks of the probability density function and to determine the number ofrate zones for tachyarrhythmia detection based on the identified peaksof the probability density function.

The cardiac device may also include a tachyarrhythmia detectorconfigured to detect tachyarrhythmia based on the number of rate zones.The cardiac device may also include a therapy circuit configured todeliver multiple tachyarrhythmia therapies, each therapy associated witha particular rate zone.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a probability function for heart rate indicating a 95%probability that the patient's heart rate will be less than or equal to220 bpm;

FIG. 1B is a flow chart illustrating a method for determining arrhythmiarate zones in accordance with embodiments of the invention;

FIG. 2 illustrates a cumulative distribution function of heart ratedeveloped based on measurements of the patient's average heart ratesthat may be used to determine the number and/or thresholds for ratezones in accordance with embodiments of the invention;

FIG. 3 illustrates a probability density function of heart ratedeveloped based on measurements of the patient's heart rate that may beused to determine the number and/or threshold values for rate zones inaccordance with embodiments of the invention;

FIG. 4 is a flow chart that illustrates a method for determining thenumber of rate zones used for tachyarrhythmia detection and/or therapyin accordance with embodiments of the invention;

FIG. 5 illustrates a cumulative distribution function that may be usedfor determining the number of rate zones for tachyarrhythmia detectionand/or therapy in accordance with embodiments of the invention;

FIG. 6 illustrates a probability density function that may be used fordetermining the number of rate zones for tachyarrhythmia detectionand/or therapy in accordance with embodiments of the invention;

FIG. 7 illustrates a two dimensional probability density function thatmay be used to determine the number and/or thresholds for rate zonesand/or LRL in accordance with embodiments of the invention;

FIG. 8 is a partial view of a cardiac rhythm management device that maybe used for determination of rate zones in accordance with embodimentsof the invention; and

FIG. 9 is a block diagram of a cardiac rhythm management device that maybe used to determine the number and/or thresholds for rate zones inaccordance with embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Some current cardiac rhythm management (CRM) devices are capable ofbeing programmed to have one or more tachyarrhythmia rate zones, each ofwhich is defined by a rate threshold. When a patient's heart rateincreases above a rate threshold, a tachyarrhythmia episode is detectedand tachyarrhythmia therapy, such as anti-tachycardia pacing (ATP),cardioversion, and/or defibrillation may be delivered. Current CRMdevices may also be programmed with a lower rate limit (LRL) forbradycardia pacing. When a patient's heart rate decreases below the LRL,the CRM device delivers pacing pulses to the heart to maintain ahemodynamically sufficient heart rate.

A physician may set the tachyarrhythmia rate thresholds and the LRL fora device at implant or may manually adjust the thresholds and/or LRLduring follow-up visits to adapt to a patient's changing medical status.Manual adjustment of rate thresholds is time consuming, and, as aresult, physicians may leave the thresholds set at their nominal values,producing sub-optimal tachyarrhythmia therapy. Furthermore, recentstudies have shown that even when physicians attempt to manuallyoptimize the tachyarrhythmia rate zone thresholds for a particularpatient, the CRM device still may not deliver optimal therapy.

The present invention is directed to methods and systems forautomatically or semi-automatically determining the number oftachyarrhythmia rate zones, tachyarrhythmia rate zone thresholds, and/orthe LRL for a CRM device. According to various embodiments,determination of the number of rate zones and/or rate zone thresholdsand the LRL is accomplished based upon a probability function for heartrate. The probability function predicts the likelihood of future heartrates based on previously measured heart rate values. For example, aprobability function for heart rate may be determined using a set ofheart rate values that have been periodically measured and stored. Ingeneral, a probability function provides the probability that avariable, which in this application is heart rate, will take on acertain value. As related to heart rate, the probability function can beused to determine the probability that the patient's heart rate willtake on a value greater than or less than a certain value. In a specificexample, based on a probability function developed from a particular setof collected and measured heart rate values (illustrated in FIG. 1A),the probability that a patient will have a heart rate less than 220 bpmis 0.95 which may be expressed as a 95% probability. According tovarious embodiments, rate zone thresholds are established from one ormore selected probability values using a probability function.

A method for determining arrhythmia rate zone thresholds is illustratedby the flowchart of FIG. 1B. A set of measured heart rate values iscollected 110, such as in a buffer in the memory of a CRM device. Aprobability function for heart rate is determined 120 based on the setof collected and measured heart rate values. One or more heart rateprobability values are selected 130. For example, the heart rateprobability values may be pre-programmed into a device, may be selectedautomatically by the device, or may be selected by a physician via anexternal programmer. One or more arrhythmia rate thresholdscorresponding to the selected probability values are determined 140based on the probability function.

According to various aspects of the invention, the rate zone thresholdsdetermined using the processes described herein may be atrial rate zonethresholds for atrial rate zones used for detection and therapy foratrial arrhythmias or may be ventricular rate zone thresholds forventricular rate zones used for detection and therapy for ventriculararrhythmias. For example, a probability function useful for determiningventricular rate zones may be developed using measured and collectedventricular rate values, or intervals between ventricular beats (R-Rintervals). A probability function useful for determining atrial ratezones may be developed using measured and collected atrial rate valuesor intervals between atrial beats (P-P intervals). In otherimplementations, the probability function may be developed usingintervals between ventricular beats and atrial beats (RP intervals)and/or intervals between atrial beats and ventricular beats (PRintervals).

The processes described herein may be used for automatic,semi-automatic, or manual initialization and/or adaptation of the apatient's tachyarrhythmia rate zone thresholds and/or LRL based on aprobability function developed using the distribution of heart rates thepatient has experienced in the past. In some embodiments, the number ofrate zones may be determined based on the probability function.Automatic, semi-automatic, or manual determination of rate zonethresholds may be incorporated into CRM devices for delivery ofappropriate, adaptable tachyarrhythmia therapy, wherein the devicelearns over time the appropriate number and thresholds fortachyarrhythmia rate zones for a particular patient. Additionally oralternatively, the processes described herein may be incorporated intoCRM devices that include pacing functionality for adjusting a lower ratelimit (LRL) for pacing when a currently used LRL is determined to besuboptimal.

In one embodiment of the invention the probability function is acumulative distribution function (CDF) created from periodicmeasurements of the patient's heart rate. In one example, the CDF,illustrated in FIG. 2, is developed based on measurements of thepatient's average heart rate taken every T minutes, where T may be equalto about one hour, about 65 minutes, or other appropriate value. Theaverage heart rate may be computed using a sample of about 10 beats orintervals, for example. Thus, according to this process, every Tminutes, an average heart rate is computed, based on a predeterminednumber of beats or intervals. In this example, the heart ratemeasurements are stored in a buffer. If the buffer is full, as a newaverage heart rate measurement comes into the buffer, the oldestmeasurement is erased. The measurements in the buffer include averagevalues obtained from intrinsic rhythm measurements, includingmeasurements taken during tachyarrhythmia or bradyarrhythmia episodes.An intrinsic bradyarrhythmia episode can occur if the LRL is set toolow.

In another example, the CDF is developed based on measurements of thetime intervals between successive heart beats, thereby creating one newheart rate measurement every heart beat. In this example, the CDF may becreated every T minutes, every T hours or every T days. The CDF iscomposed of all the time intervals that occur within those T minutes,hours or days so that short tachyarrhythmia episodes are not missed oraveraged out.

A CDF, as illustrated in FIG. 2, may be updated every T minutes, forexample, using the current measurements stored in the buffer. FIG. 2illustrates an idealized case where the average heart rate measurementsare normally distributed. Alternatively, the probability function usedto determine the rate zone thresholds may be a probability densityfunction (PDF) as illustrated in FIG. 3. In FIGS. 2 and 3, thehorizontal axes represent the range of measured heart rate values. InFIG. 2, the vertical axis is the fraction of measurements stored in thebuffer that are less than a particular heart rate value. In FIG. 3, thevertical axis is the fraction of measurements stored in the buffer thatare equal to a particular heart rate value. In the example providedbelow, determination of rate zone thresholds is illustrated using theCDF, although the PDF (or other probability functions) may alternativelybe used.

With reference to FIG. 2, there is a probability value, P_(hi),corresponding to a fraction of heart rate measurements that are lessthan a particular heart rate value, T_(hi). Similarly, there is aprobability value, P_(lo), corresponding to a fraction of heart ratemeasurements that are less than a particular heart rate value, T_(lo).Heart rate values within the range T_(lo)-T_(hi) are considered to benormal sinus rhythm (NSR). Heart rates greater than T_(hi) areconsidered tachyarrhythmic, while heart rates less than T_(lo) areconsidered bradyarrhythmic.

At implant or at any clinic visit, rather than directly specifying heartrate values for the rate zone thresholds or LRL, a physician may insteadselect P_(hi) and/or P_(lo). P_(hi) is used to specify thetachyarrhythmia threshold rate value T_(hi). P_(lo) is used to specifythe bradyarrhythmia threshold value, or lower rate limit (LRL) T_(lo).In various embodiments, P_(hi) and/or P_(lo) may also be pre-programmedinto the device as nominal values or may be selected by the physician atthe time of implant or subsequent to implant. Based on the values ofP_(hi) and/or P_(lo), the device uses the CDF to determine thecorresponding values for T_(hi) and/or T_(lo) for that particularpatient. Thus, the probability function serves as a functional “look-uptable” with the probability values as indexes for determining values forthe tachyarrhythmia rate zone threshold, T_(hi), and/or the LRL, T_(lo),that correspond to the selected values of P_(hi) and P_(lo),respectively. A rate threshold may be determined by inputting aprobability value into the probability function equation and solving theequation to determine the rate threshold.

Use of a probability function and specification of probability valuesrather than rates to determine rate thresholds allows the rate zonethresholds to adapt over time. For example, if a physician selects aparticular heart rate as a rate zone threshold, that rate zone thresholdis static over time. However, when the rate zone threshold is specifiedas a probability value, rather than a static heart rate, the rate of therate zone threshold is dynamic. As the set of measured values used todevelop the probability function shifts, the probability value staysconstant but the threshold rate corresponding to the probability valuechanges along with the probability function.

The use of the probability function in determining rate zone thresholdsallows the physician to more easily compare the effects oftachyarrhythmia rate zone thresholds across patients. For example, arelatively low tachyarrhythmia rate threshold may be optimum for aninactive patient but can be severely inadequate for an active patient.As a specific example, consider an elderly, inactive patient having NSRthat varies from 60-90 bpm. A tachyarrhythmia rate zone threshold of 130may be adequate for this patient. However, consider a patient whose NSRvaries from 60-150 bpm. Setting a tachyarrhythmia rate zone thresholdfor this patient at 130 may result in frequent delivery of inappropriateanti-tachyarrhythmia therapy.

However, using the principles of the invention, selecting a probabilityvalue of 99% would yield an appropriate tachyarrhythmia rate zonethreshold value for both of these patients. A P_(hi) selected as 99.9%will classify as tachyarrhythmic the highest 0.1% of a patient's heartrate measurements, regardless of whether the patient's NSR ranges from60-90 bpm or 60-150 bpm.

According to some embodiments, multiple rate zones may be specifiedusing the processes described herein. In one example, the physiciancould specify multiple values of P_(hi), such as P_(hi1)=99.5%,P_(hi2)=99.7%, and P_(hi3)=99.9%. The device determines values forT_(hi1), T_(hi2), T_(hi3), respectively corresponding to P_(hi1),P_(hi2), and P_(hi3), such as through the use of a CDF or a PDF. Alowest tachyarrhythmia rate zone (VT-1) would span the range of ratesT_(hi1)-T_(hi2), the next tachyarrhythmia rate zone (VT-2) would spanthe range T_(hi2)-T_(hi3), and the fibrillation rate zone (VF) wouldinclude rates greater than T_(hi3).

Some patients require only one tachyarrhythmia rate zone while otherpatients may need two or more. Multiple rate zones may be advantageousfor patients that experience several types of arrhythmias, wherein eacharrhythmia occurs at a different rate. Different types of arrhythmiasmay be best treated using different therapy schemes. Some embodiments ofthe invention are directed to methods and devices for determining thenumber of tachyarrhythmia rate zones for a particular patient.

As illustrated by the flow chart of FIG. 4, the number of rate zonesused for tachyarrhythmia detection and/or therapy may be determinedbased on a probability function for heart rate. A set of measured heartrate values is collected 410. The probability function for heart rate isdeveloped 420 using the set of collected and measured heart rate values.The number of rate zones for arrhythmia detection is determined 430based on the shape of the probability function.

FIGS. 5 and 6 illustrate determining the number of rate zones based onthe shape of the probability function. FIG. 5 illustrates the processbased on a CDF with FIG. 6 showing a corresponding PDF. The CDF of FIG.5 exhibits two relatively flat portions 510, 520. The flat portions 510,520 indicate separation between peaks 610, 620 of the PDF shown in FIG.6. The number of the flat portions 510, 520 of the CDF (FIG. 5) or thenumber of peaks 610, 620 of the PDF (FIG. 6) may be used to determinethe number of rate zones needed for the patient. For example, the CDFshown in FIG. 5 indicates that tachyarrhythmia detection and/or therapymay be enhanced by using two additional rate zone thresholds. A firstrate zone is associated with the fraction of measured ratescorresponding to P_(hi1), a second rate zone is indicated by the firstflat portion 510, the first flat portion associated with the fraction ofmeasured rates corresponding to P_(hi2), and a third rate zone indicatedby the second flat portion 520 associated with the fraction of measuredrates corresponding to P_(hi3). Threshold values, T_(hi1), T_(hi2), and,T_(hi3), respectively associated with P_(hi1), P_(hi2), and P_(hi3), areselected as the threshold values for a first rate zone 1, VT-1, a secondrate zone, VT-2, and a third rate zone, VF.

After detection of a tachyarrhythmia episode by use of the rate zonesVT-1, VT-2, and VF, an appropriate therapy may be delivered to terminatethe arrhythmia. For example, a first type of therapy may be delivered totreat tachyarrhythmia episodes having rates that fall within rate zoneVT-1, a second type of therapy may be delivered to treat tachyarrhythmiaepisodes that fall within rate zone VT-2, and a third type of therapymay be delivered to treat tachyarrhythmia episodes that fall within ratezone VF.

As previously discussed, the processes for determining thetachyarrhythmia rate zone threshold values, the LRL, and/or the numberof tachyarrhythmia rate zones based on a probability function of heartrate values may be implemented fully automatically, semi-automatically,or manually. The tachyarrhythmia rate zones determined by the processesdescribed herein may be used by a CRM device to detect and treatventricular or atrial tachyarrhythmias.

In one embodiment, the CRM device may use a multi-dimensional CDF fordetermination of the number and/or threshold values of rate zones. Forexample, the CRM device may measure store both atrial and ventricularheart rates. From these values, a two dimensional CDF or PDF can bedeveloped, with one dimension representing the atrial heart rate and asecond dimension representing the ventricular rate. A two-dimensionalPDF is graphically illustrated in FIG. 7 wherein the shading is used toindicate probability values of the PDF. The two dimensional PDF may beused to incorporate trends in the atrial rate into the automated choiceof tachyarrhythmia rate zone thresholds and the LRL. For example, asillustrated by FIG. 7, the rate zone threshold may be multi-dimensional,having an atrial threshold, T_(hi) _(—a) , and a ventricular threshold,T_(hi) _(—v) . Tachyarrhythmia is detected, and therapy may bedelivered, based on both the atrial and ventricular thresholds. The twodimensional PDF may also be used to determine a number of the rate zonesfor tachyarrhythmia detection. As in the one-dimensional case, the shapeof the two dimensional PDF may be used to determine the number of ratezones that are optimal for a particular patient. The use of a twodimensional PDF based on both atrial and ventricular rates isparticularly advantageous for patients suffering from chronic atrialtachyarrhythmia where high atrial rhythms affect the patient'sventricular rate.

FIG. 8 illustrates a view of a CRM device 800 incorporating circuitrycapable of implementing processes for determining the number oftachyarrhythmia rate zones, tachyarrhythmia rate zone thresholds, and/orLRL based on a probability function of heart rate as described herein.The CRM device 800 includes circuitry enclosed within an implantablehousing 801 and electrically coupled to an intracardiac lead system 810.

Portions of the intracardiac lead system 810 are inserted into thepatient's heart. The lead system 810 includes cardiac pace/senseelectrodes 851-856 positioned in, on, or about one or more heartchambers for sensing electrical signals from the patient's heart and/ordelivering pacing pulses to the heart. The intracardiac sense/paceelectrodes 851-856, such as those illustrated in FIG. 7, may be used tosense and/or pace one or more chambers of the heart, including the leftventricle, the right ventricle, the left atrium and/or the right atrium.The lead system 810 also includes defibrillation coils 841, 842. The CRMcircuitry controls electrical stimulation delivered via the electrodes841, 842, 851-856. The electrical stimulation may be delivered in theform of ATP or cardioversion/defibrillation shocks to interrupttachyarrhythmic episodes. The electrical stimulation may be delivered inthe form of relatively low energy pacing pulses to ensure that the heartbeats at a hemodynamically sufficient rate.

The lead system 810 may include a left ventricular lead systemincorporating electrodes 854 and 855 positioned within the coronaryvenous system proximate the left ventricle. Stimulating the ventricle atmultiple locations in the left ventricle or at a single selectedlocation may provide for increased cardiac output in a patientssuffering from congestive heart failure (CHF), for example, and/or mayprovide for other benefits.

Portions of the housing 801 of the CRM device 800 may optionally serveas one or multiple can or indifferent electrodes. The housing 801 isillustrated as incorporating a header 889 that may be configured tofacilitate removable attachment between one or more leads of the leadsystem 810 and the housing 801. The housing 801 of the CRM device 800may include one or more can electrodes that may be used for pacingand/or cardioversion/defibrillation.

The CRM device illustrated in FIG. 8 includes circuitry for measuringheart rate 860, a memory for storing heart rate measurements 865, andcircuitry 870 for determining the number and/or threshold values for oneor more tachyarrhythmia rate zones based on a probability function forheart rate. The heart rate measurement circuitry 860, memory 865, andrate zone circuitry 870 may also be used to determine the LRL forbradyarrhythmia pacing as described herein.

Communications circuitry is disposed within the housing 801 forfacilitating communication between the CRM device 800 and apatient-external device, such as an external programmer or advancedpatient management (APM) system. Determination of the number of ratezones and/or rate zone thresholds may be performed automatically by thedevice or may be performed semi-automatically using a probability valuereceived from a physician via the patient-external programmer.

In an example of semi-automatic operation, the CRM device 800 mayperiodically measure heart rate and collect heart rate measurements in amemory buffer. The CRM device 800 then develops a probability functionof heart rate based on the collected and measured heart ratemeasurements. Via the programmer, a physician can enter a probabilityvalue used for determining a rate zone threshold. For example, thephysician may specify that the rate zone threshold be set at valuecorresponding to a fraction of the set of measured and collected heartrate measurements, such as 95%. The CRM device 800 selects the heartrate value from the probability function that is greater than 95% of themeasurements included in buffer as the rate zone threshold.

FIG. 9 is a block diagram of a CRM device 900 incorporating circuitryfor determining the number and/or thresholds for rate zones inaccordance with embodiments of the invention. The CRM device 900includes pacing therapy circuitry 930 that delivers pacing pulses to aheart. The CRM device 900 may optionally includedefibrillation/cardioversion circuitry 935 configured to deliver highenergy defibrillation or cardioversion stimulation to the heart forterminating dangerous tachyarrhythmias.

The pacing and/or defibrillation pulses may be delivered via multiplecardiac electrodes 905 disposed at multiple locations within a heart.The electrodes 905 are coupled to switch matrix 925 circuitry used toselectively couple electrodes 905 of various pacing vectors to sensingand therapy circuitry 910, 930, 935.

The sensing/detection circuitry 910 in cooperation with sensingelectrodes 905 detect electrical signals produced by heart. From theheart's sensed electrical signals, the heart rate measurement circuitry915 periodically, e.g., about every hour or about every 65 minutes,determines the average heart rate or average interval between beats. Forexample, the heart rate measurement circuitry 915 may measure the atrialrate or P-P intervals and/or may measure the ventricular rate or R-Rintervals. Measurements of heart rate (or heart rate intervalmeasurements) are stored in a circular buffer in the memory 945.

Periodically, the rate zone processor 960 accesses the memory buffer andcalculates a probability function for heart rate from the measurementvalues stored in the buffer. The rate zone processor 960 may determinethe number of rate zones that are appropriate to enhance tachyarrhythmiadetection and/or therapy delivered to the patient.

A probability value used for determining a rate zone threshold may bedetermined either by the CRM device 900 or may be entered by a physicianfrom a patient external device 955 via communications circuitry 950. Therate zone processor 960 uses the probability value determine the ratezone threshold value based on the probability function. In certainimplementations, the rate zone processor 960 may also determine ormodify the LRL used for pacing based on the probability function.

The rate zone processor 960 may determine one or more rate zonethresholds, may determine a LRL, and may determine the number of ratezones. The arrhythmia detector 920 uses the rate zone thresholds fordetecting arrhythmia. For example, the arrhythmia detector may compare acurrent heart rate to the rate zone thresholds and may determine thattachyarrhythmia is occurring if the heart rate exceeds a rate zonethreshold. If tachyarrhythmia occurs, the therapy control processor 940may control the delivery of therapy to mitigate the tachyarrhythmia. IfThe therapy control processor may have the capability to deliverymultiple types of therapies, involving anti-tachyarrhythmia pacing(ATP), cardioversion and/or defibrillation. These multiple types ortherapies may be associated respectively with rate zones. Where multipletherapies respectively associated with multiple rate zones are used, thetherapy control processor 940 controls delivery of a therapy associatedwith the particular rate zone of the arrhythmia. The therapy controlprocessor may also control delivery of pacing pulses to maintain pacingabove the LRL.

An cardiac device according to the present invention may include one ormore of the features, structures, methods, or combinations thereofdescribed herein. For example, a cardiac device may be implemented toinclude one or more of the advantageous features and/or processesdescribed. It is intended that such a implanted, partially implanted, orpatient-external device need not include all of the features describedherein, but may be implemented to include selected features that providefor useful structures and/or functionality.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method for detecting cardiac arrhythmia; comprising: collecting measured heart rate values; determining a probability function for heart rate using the collected and measured heart rate values; selecting one or more heart rate probability values; and determining one or more rate zone thresholds for arrhythmia rate zones from the probability function based on the one or more probability values.
 2. The method of claim 1, further comprising detecting cardiac tachyarrhythmia based on the rate zone thresholds.
 3. The method of claim 1, wherein: the collected and measured heart rate values comprise RR intervals; determining the probability function comprises determining the probability function using the RR intervals; and determining the one or more rate zone thresholds comprises determining one or more ventricular rate thresholds.
 4. The method of claim 1, wherein: the collected and measured heart rate values comprise PP intervals; determining the probability function comprises determining the probability function using the PP intervals; and determining the one or more rate zone thresholds comprises determining one or more atrial rate thresholds.
 5. The method of claim 1, wherein the probability function comprises a cumulative density function.
 6. The method of claim 1, wherein selecting the one or more probability values comprises entering one or more fractional probability values via a patient-external device.
 7. The method of claim 1, wherein determining the one or more rate thresholds comprises using the probability function as a lookup table and the one or more probability values as indexes to the lookup table.
 8. The method of claim 1, wherein determining the one or more rate thresholds comprises solving the probability function by inputting the one or more probability values into the equation to determine the one or more rate thresholds.
 9. The method of claim 1, wherein determining the one or more rate zone thresholds comprises determining a threshold for a lower rate limit.
 10. The method of claim 1, wherein determining the one or more rate zone thresholds comprises determining one or more tachyarrhythmia rate zone thresholds.
 11. The method of claim 1, further comprising determining a number of the arrhythmia rate zones based on the probability function.
 12. The method of claim 1, wherein the probability function comprises a one dimensional probability function of ventricular rate values.
 13. The method of claim 1, wherein the probability function comprises a one dimensional probability function of atrial rate values.
 14. The method of claim 1, wherein the probability function comprises a two dimensional probability function of atrial rate values and ventricular rate values.
 15. The method of claim 14, comprising: identifying atrioventricular patterns based on the two dimensional probability function of atrial rate values and ventricular rate values; and determining the one or more rate zone thresholds comprises determining the rate zone thresholds based on the two dimensional probability function.
 16. The method of claim 1, wherein determining the one or more rate zone thresholds comprises determining multiple rate thresholds respectively associated with multiple rate zones.
 17. The method of claim 16, further comprising: detecting arrhythmia based on the multiple rate zones; and delivering cardiac stimulation therapy to treat the arrhythmia.
 18. The method of claim 17, wherein delivering the cardiac stimulation therapy comprises delivering multiple therapies, each therapy respectively associated with one of the multiple rate zones.
 19. A cardiac device, comprising: sensing circuitry configured to sense cardiac electrical signals; heart rate measurement circuitry coupled to the sensing circuitry and configured to measure heart rate values based on the sensed cardiac electrical signals; a memory buffer configured to store the measured heart rate values; a rate zone processor configured to develop a probability function for heart rate based on the measured heart rate values and to determine at least one rate zone threshold from the probability function based a heart rate probability value.
 20. The device of claim 19, wherein the rate zone processor is configured to automatically select the heart rate probability value.
 21. The device of claim 19, further comprising communication circuitry configured to receive the heart rate probability value entered via a patient-external device.
 22. The device of claim 19, wherein the rate zone processor is configured to use the probability function as a look up table to determine the rate zone threshold using the heart rate probability value as an index.
 23. The device of claim 19, wherein the rate zone processor is configured to solve the probability function using the heart rate probability value to determine the rate zone threshold.
 24. The device of claim 19, wherein: the heart rate measurement circuitry is configured to measure intervals between cardiac beats; and the rate zone processor is configured to develop the probability function based on the measured intervals.
 25. The device of claim 19, wherein the probability function comprises a cumulative density function.
 26. The device of claim 19, wherein the rate zone processor is further configured to determine a number of rate zones based on the probability function.
 27. The device of claim 19, wherein the probability function comprises a multi-dimensional probability function of atrial and ventricular heart rate values.
 28. The device of claim 19, further comprising: detection circuitry configured to detect arrhythmia based on the rate zone threshold; and therapy circuitry configured to deliver electrical stimulation therapy to treat the arrhythmia.
 29. A method for detecting cardiac arrhythmia, comprising: collecting measured heart rate values; determining a probability function for the collected and measured heart rate values; and determining a number of rate zones for tachyarrhythmia detection based on the probability function.
 30. The method of claim 29, wherein determining the number of rate zones comprises: detecting one or more features of the probability function; and determining the number of rate zones based on a number of the one or more features.
 31. The method of claim 29, wherein: the probability function comprises a cumulative distribution function; and determining the number of rate zones comprises analyzing the cumulative distribution function to identify flat portions of the cumulative distribution function.
 32. The method of claim 29, wherein: the probability function comprises a probability density function; and determining the number of rate zones comprises analyzing the probability density function to identify peaks of the probability density function.
 33. The method of claim 29, further comprising periodically updating the number of rate zones.
 34. A cardiac device, comprising: sensing circuitry configured to sense cardiac electrical signals; heart rate measurement circuitry coupled to the sensing circuitry and configured to collect and measure heart rate values based on the sensed cardiac electrical signals; and a rate zone processor configured to determine a number of rate zones for tachyarrhythmia detection based on a probability function for the collected and measured heart rate values.
 35. The device of claim 34, wherein: the probability function comprises a cumulative distribution function; and the rate zone processor is configured to analyze the cumulative distribution function to identify flat portions of the cumulative distribution function and to determine the number of rate zones for tachyarrhythmia detection based on the identified flat portions of the cumulative distribution function.
 36. The device of claim 34, wherein: the probability function comprises a probability density function; and the rate zone processor is configured to analyze the cumulative distribution function to identify peaks of the probability density function and to determine the number of rate zones for tachyarrhythmia detection based on the identified peaks of the probability density function.
 37. The device of claim 34, further comprising a tachyarrhythmia detector configured to detect tachyarrhythmia based on the number of rate zones.
 38. The device of claim 34, further comprising a therapy circuit configured to deliver multiple tachyarrhythmia therapies, each therapy associated with a particular rate zone. 